A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In present lithographic apparatus, a device for transmission image detection is used in order to align a reticle to a wafer stage. It consists of a structure (e.g. a grating) on a reticle and a complementary structure on a transmission image detector plate. The image of the structure is scanned using the transmission image detector to determine position and focus of the image. The transmission image detector has a small number (typically 4-8) of such structures. Beneath each structure a photodiode is located to detect the light. The transmission image detector is conventionally used to measure first-order positioning terms like translation, magnification and rotation of the reticle with respect to the wafer stage. Higher order distortions remain unresolved. Such higher order distortions may arise as a consequence of reticle heating and/or lens heating. Any distortions, including these higher order distortions may lead to overlay errors. For high throughput machines these higher order distortions will become one of the largest posts in the overlay budget.
In device manufacturing methods using lithographic apparatus, overlay is an important factor in the yield, i.e. the percentage of correctly manufactured devices. Overlay is the accuracy within which layers are printed in relation to layers that have previously been formed. The overlay error budget will often be 10 nm or less, and to achieve such accuracy, the substrate must be aligned to the mask pattern to be transferred with great accuracy.